A generally used type of lithographic printing plate precursor has a radiation sensitive coating applied to an aluminum substrate. A positive working precursor has a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. Only the remaining, image, area of the coating is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing plate precursor with a vacuum to ensure good contact. The printing plate precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive printing plate precursor is used, the area of the film that corresponds to the image in the printing plate precursor is opaque so that no light will strike the printing plate precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development.
In the manufacture of electronic parts such as printed circuits, after exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements--for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive-working systems are widely used. In particular, in the main there have been used alkali developable positive working resists mainly composed of alkali-soluble novolac resins.
The types of electronic parts whose manufacture may use a resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
Imagable compositions may also be applied to plastics films in order to form masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing plate or electronic part precursor.
Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and naphthoquinone diazide (NQD) derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulphonyl chloride.
As demands on the performance of UV sensitive positive working coatings have increased so NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on coatings.
We have devised new positive working heat sensitive systems, to meet the new demands. Our new systems and methods are the subject of our patent applications WO 97/39894, WO 99/01796, WO 99/01795, WO 99/08879, WO 99/21715, WO 99/21725 and WO 99/11458 which are hereby incorporated by reference in their entirety. Heat is delivered to the coatings described by conduction, using a heated body such as a stylus, or by charged particle radiation, or, preferably, by means of infra-red radiation, the coatings then containing suitable infra-red absorbers.
Our new systems are very effective and are in production, and proving very successful in the marketplace, but it would be desirable to improve them further, by making their coatings more resistant to the effects of movement; for example within the manufacturing plant; during transportation from the manufacturing plant to a customer's site; and within the customer's site. One example of a source of problems in the customer's site is at the image setter. This will often have linear conveying equipment to convey a precursor into and out of the image setter, and this can leave markings in the travel direction, and/or it may have lifting equipment which can leave markings caused by elastomeric lifting disks.
It is an object of the invention to provide articles having coatings which may be imaged by heat, the articles having good resistance to the effects of movement, especially caused by mechanical handling equipment, notably such equipment used by printers and printed circuit board manufacturers.